We propose to identify the enzymic defects which make human beings and animals dependent on a dietary supply of the so-called "essential amino acids" (EAA). The human requirement for protein is not a requirement for protein per se. It is actually a requirement for a balanced mixture of EAA plus a nitrogen source. Why certain amino acids are essential in animal and human diet is unknown. Does the requirement for an EAA result from lack of a single enzyme needed for its biosynthesis? Or, is the entire metabolic pathway missing from animal and human tissue? Answers to these questions are fundamental to the understanding of the growing problem of human protein malnutrition. They may also have practical applications in devising low-nitrogen diets for human nitrogen accumulation diseases and in the assessment of adequacy of protein nutrition. The possibility that lack of a single enzyme results in a dietary requirement for a given EAA is of particular interest. Perhaps precursors of that EAA accumulate in animal tissue when the EAA is withdrawn from diet. (The situation would be analogous to that in microorganisms in which the product of a biosynthetic pathway is needed to control the rate of its biosynthesis by feedback inhibition or by repression or by both mechanisms.) Accumulation of EAA precursors in EAA deficient animals and humans might explain the characteristic symptoms of EAA deficiency: severe appetite depression and, in the case of valine-deficient rats of isoleucine-deficient humans, neurolocal symptoms. We propose: (1) to determine to what extent rats and humans have retained the enzymic machinery necessary for synthesis of EAA, (2) to test the ability of tissue slices and crude enzyme preparations to form EAA precursors from radioactive substrates, (3) to examine urine, blood and other tissues of EAA-deficient and control animals for EAA precursors, (4) to investigate the neuropathology of EAA deficiency at the subcellular level using electron microscopy, and (5) to explore the relationship of any detectable EAA precursors to the appetite failure and neurological damage characteristic of EAA deficiency.